Probing QCD critical fluctuations from the yield ratio of strange hadrons in relativistic heavy-ion collisions

TL;DR

This study analyzes strange hadron yield ratios in heavy-ion collisions across various energies, revealing potential non-monotonic behavior indicative of QCD critical fluctuations, and highlights the importance of quark density fluctuations near the critical endpoint.

Contribution

It introduces a quark coalescence model incorporating quark density fluctuations to explain non-monotonic energy dependence of strange hadron ratios, suggesting a signal for the QCD critical endpoint.

Findings

Possible non-monotonic behavior in strange hadron yield ratio with collision energy.

Quark density fluctuations may cause observable signals near the QCD critical point.

Standard models fail to reproduce the observed energy dependence of the ratio.

Abstract

By analyzing the available data on strange hadrons in central Pb+Pb collisions from the NA49 Collaboration at the Super Proton Synchrotron (SPS) and in central Au+Au collisions from the STAR Collaboration at the Relativistic Heavy-Ion Collider (RHIC) in a wide collision energy range from $\sqrt{s_{\rm NN}}$ = 6.3 GeV to 200 GeV, we find a possible non-monotonic behavior in the ratio $\mathcal{O}_\text{K-$\Xi$-$\phi$-$\Lambda$}$= $\frac{N(K^+)N(\Xi^-)}{N(\phi)N(\Lambda)}$ of $K^+$, $\Xi^-$, $\phi$, and $\Lambda$ yields as a function of $\sqrt{s_{\rm NN}}$. Based on the quark coalescence model, which can take into account the effect of quark density fluctuations on hadron production, a possible non-monotonic behavior in the dependence of the strange quark density fluctuation on $\sqrt{s_{NN}}$ is obtained. This is in contrast to the coalescence model that does not include quark density fluctuations and also to the statistical hadronization model as both fail to describe even qualitatively the collision energy dependence of the ratio $\mathcal{O}_\text{K-$\Xi$-$\phi$-$\Lambda$}$. Our findings thus suggest that the signal and location of a possible critical endpoint in the QCD phase diagram, which is expected to result in large quark density fluctuations, can be found in the on-going Bean Energy Scan program at RHIC.